This invention pertains to a propulsion system for supersonic aircraft.
Since the flight envelope of a supersonic aircraft is very large, the propulsion engine cannot be optimized in every case and the properties required of it may even be contradictory. In particular, the principal obstacle to the development of the supersonic transport is the problem of take-off noise. It is essential that future supersonic transport aircraft satisfy not only existing regulations on take-off noise levels but also those being drafted at this time. This requirement, which entails a moderate ejection velocity, is consistent with the requirements of adequate low-speed efficiency, but can hardly be compatible with obtaining a small nacelle cross-section and good efficiency at supersonic speeds.
The principal characteristics required of the propulsion plant at supersonic cruising speeds are a low specific fuel consumption rate and a high specific thrust (thrust per unit of fuel flow rate).
The thrust furnished to the air frame by the propulsion system is the resultant of the engine thrust and the drag of the nacelle, primarily the wave drag, which represents a not negligible portion of the total drag of the aircraft.
Optimization should thus concern the entirety of the propulsion plant, rather than the engine alone.
The forebody drag is proportional to the cross-sectional area of the nacelle and depends on the difference between the sectional areas of the air intake and of the maximum cross sectional area of the nacelle.
As a first approximation, it may be said that the largest section of the nacelle is proportional to the engine intake cross-sectional area.
On take-off, the propulsion plant should have a low ejection velocity so as to reduce noise. In order to furnish satisfactory thrust, the propulsion plant thus requires a significant intake flow rate and, consequently, a large intake cross section. This requirement is hardly compatible with the problem posed by supersonic cruising.
Combined aircraft propulsion systems have already been proposed which provide wide ranges of speed and thrust by associating one or several turbojet engines with a by-pass turbojet engine.
Nevertheless, these known combination systems, which rely on a tandem or parallel arrangement, do not allow satisfactory aerodynamic configurations.